Humidifying apparatus such as disclosed in U.S. Pat. Nos. 4,110,419, 4,172,105, 4,366,105, 4,500,480, 4,674,494, 4,765,327 and 6,050,552 describe different variations of cartridge-type humidification systems and apparatus incorporating a heated humidifier module connected via tubing to a water supply reservoir. In these systems, the humidification chamber is installed within a heater receptacle. As water is evaporated in the heated humidification chamber replacement water is fed to the cavity from the adjacent water supply reservoir. The descriptions of the aforesaid patents are incorporated herein by reference.
In U.S. Pat. Nos. 4,500,480 and 6,050,552, a water level sensing tube positioned in the humidification chamber communicates with the air space above the water level in the water supply reservoir via tubing with a one-way valve. Water is fed to the humidification chamber from the bottom of the reservoir via tubing, also provided with a one-way valve. During continuous gas flow, for example, where the humidification apparatus is used with a CPAP system or is connected to a respirator or ventilator which directs respiratory gas into the humidification chamber under pressure, the pressure in the water supply reservoir equals the pressure to which the humidification chamber is exposed. Under certain conditions, the humidification cartridge may be exposed to pressures of 150 cm H2O or more during continuous gas flow, or pressures in excess of 120 cm H2O during intermittent gas flow. The high pressure is transferred ultimately to the reservoir via the one-way tube so that the pressure inside the reservoir is near or equal to the mean pressure to which the humidification chamber is exposed. During typical operation, such high pressures in the containers of the system are not problematic. However, sudden pressure reduction in the humidification chamber can result in rapid transfer of water from the reservoir and flooding of the chamber and even into the respiratory gas tubing to a patient. The apparatus described herein is designed to substantially reduce or eliminate such flooding.